1. Field of the Invention
This application is directed to microwave ovens and to mode stirrers for use in microwave ovens and, more particularly, to a novel drive arrangement for rotating a mode stirrer device in the path of microwave energy entering the cooking cavity of a microwave oven.
2. Description of the Prior Art
One well known problem associated with conventional microwave ovens concerns the uneven distribution of microwave energy within the cooking cavity. The result of such unevenness has been the creation of "hot spots" and "cold spots" at different finite areas of the oven. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When such a standing wave pattern is set up, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern during a cooking cycle is dependent on a multitude of factors, among which are the characteristics of the microwave energy source, the dimensions and makeup of the cavity, and the loading effect of different types and quantities of food which are placed in the cooking cavity.
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried with varying degrees of success. The most common approach involves the use of a so-called "mode-stirrer" or mode changer which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwave ovens) or, to conserve space within the cooking cavity, may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The most common method of turning the blades of such mode stirrers is to attach the blades to a shaft which extends into the microwave oven cooking cavity through the top wall of the enclosure. An electric motor is coupled to the end of the shaft opposite the blades to impart a rotary motion thereto (U.S. Pat. Nos. 3,783,219 and 2,813,185). This arrangement has as its main disadvantage the need for a separate motor to drive the mode stirrer blades. Additionally, care must be taken to prevent the microwave energy from escaping through the shaft opening to damage or destroy the motor.
Yet another prior art arrangement is shown in U.S. Pat. Nos. 4,019,010; 3,471,671; and 3,991,295, in which the mode stirrer blades themselves are driven by an air stream used to cool the oven magnetron. The advantage of this arrangement is that it does away with the need for a separate motor by utilizing an already present air stream used to cool the magnetron. However, by passing the air stream over the mode stirring blades themselves severe restrictions are placed on the shape and angular orientation of these blades. More specifically, since the blades driven by the air stream also perform the additional function of changing modes within the oven cavity, a compromise blade design which satisfactorily performs both functions, albeit each in a less than optimum manner, must be used. Secondly, this design ordinarily requires the use of an air flow path through the oven cavity itself, and this presents an additional variable to deal with in properly designing the oven by requiring a minimum amount of air circulation to obtain satisfactory mode mixing.
The aforementioned Bale application, Ser. No. 107,003, addresses the above problems and provides for a mode stirrer arrangement wherein the stirrer is driven by means of an air flow which does not impinge on the mode stirring blades themselves. The mode stirrer blades are carried on one end of a rotatable shaft and positioned in the path of microwave energy entering the oven cooking cavity. The other end of the shaft carries a drive fan which is separated from the mode stirrer blades by an air impervious wall and a stream of air is directed onto the drive fan to thereby rotate the shaft and mode stirrer blades. The stream of air onto the drive fan is provided via an air channel which communicates with the magnetron cooling blower.
However, the arrangement of the aforementioned application was found to require a relatively high volume air flow to drive the mode stirrer and resulted in relatively high noise levels during rotation of the mode stirrer assembly.